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.\" $FreeBSD$
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.Dd January 5, 2010
.Dt CARP 4
.Os
.Sh NAME
.Nm carp
.Nd Common Address Redundancy Protocol
.Sh SYNOPSIS
.Cd "device carp"
.Sh DESCRIPTION
The
.Nm
interface is a pseudo-device that implements and controls the
CARP protocol.
CARP allows multiple hosts on the same local network to share a set of IP addresses.
Its primary purpose is to ensure that these
addresses are always available, but in some configurations
.Nm
can also provide load balancing functionality.
.Pp
A
.Nm
interface can be created at runtime using the
.Nm ifconfig Li carp Ns Ar N Cm create
command or by configuring
it via
.Va cloned_interfaces
in the
.Pa /etc/rc.conf
file.
.Pp
To use
.Nm ,
the administrator needs to configure at minimum a common virtual host ID (VHID)
and virtual host IP address on each machine which is to take part in the virtual
group.
Additional parameters can also be set on a per-interface basis:
.Cm advbase
and
.Cm advskew ,
which are used to control how frequently the host sends advertisements when it
is the master for a virtual host, and
.Cm pass
which is used to authenticate
.Nm
advertisements.
The
.Cm advbase
parameter stands for
.Dq "advertisement base" .
It is measured in seconds and specifies the base of the advertisement interval.
The
.Cm advskew
parameter stands for
.Dq "advertisement skew" .
It is measured in 1/256 of seconds.
It is added to the base advertisement interval to make one host advertise
a bit slower that the other does.
Both
.Cm advbase
and
.Cm advskew
are put inside CARP advertisements.
These configurations can be done using
.Xr ifconfig 8 ,
or through the
.Dv SIOCSVH
.Xr ioctl 2 .
.Pp
Additionally, there are a number of global parameters which can be set using
.Xr sysctl 8 :
.Bl -tag -width ".Va net.inet.carp.arpbalance"
.It Va net.inet.carp.allow
Accept incoming
.Nm
packets.
Enabled by default.
.It Va net.inet.carp.preempt
Allow virtual hosts to preempt each other.
It is also used to failover
.Nm
interfaces as a group.
When the option is enabled and one of the
.Nm
enabled physical interfaces
goes down,
.Cm advskew
is changed to 240 on all
.Nm
interfaces.
See also the first example.
Disabled by default.
.It Va net.inet.carp.log
Value of 0 disables any logging.
Value of 1 enables logging state changes of
.Nm
interfaces.
Values above 1 enable logging of bad
.Nm
packets.
Default value is 1.
.It Va net.inet.carp.arpbalance
Balance local traffic using ARP (see below).
Disabled by default.
.It Va net.inet.carp.suppress_preempt
A read only value showing the status of preemption suppression.
Preemption can be suppressed if link on an interface is down
or when
.Xr pfsync 4
interface is not synchronized.
Value of 0 means that preemption is not suppressed, since no
problems are detected.
Every problem increments suppression counter.
.El
.Sh ARP level load balancing
The
.Nm
has limited abilities for load balancing the incoming connections
between hosts in Ethernet network.
For load balancing operation, one needs several CARP interfaces that
are configured to the same IP address, but to a different VHIDs.
Once an ARP request is received, the CARP protocol will use a hashing
function against the source IP address in the ARP request to determine
which VHID should this request belong to.
If the corresponding CARP interface is in master state, the ARP request
will be replied, otherwise it will be ignored.
See the
.Sx EXAMPLES
section for a practical example of load balancing.
.Pp
The ARP load balancing has some limitations.
First, ARP balancing only works on the local network segment.
It cannot balance traffic that crosses a router, because the
router itself will always be balanced to the same virtual host.
Second, ARP load balancing can lead to asymmetric routing
of incoming and outgoing traffic, and thus combining it with
.Xr pfsync 4
is dangerous, because this creates a race condition between
balanced routers and a host they are serving.
Imagine an incoming packet creating state on the first router, being
forwarded to its destination, and destination replying faster
than the state information is packed and synced with the second router.
If the reply would be load balanced to second router, it will be
dropped due to no state.
.Sh EXAMPLES
For firewalls and routers with multiple interfaces, it is desirable to
failover all of the
.Nm
interfaces together, when one of the physical interfaces goes down.
This is achieved by the preempt option.
Enable it on both host A and B:
.Pp
.Dl sysctl net.inet.carp.preempt=1
.Pp
Assume that host A is the preferred master and 192.168.1.x/24 is
configured on one physical interface and 192.168.2.y/24 on another.
This is the setup for host A:
.Bd -literal -offset indent
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.2.1/24
.Ed
.Pp
The setup for host B is identical, but it has a higher
.Cm advskew :
.Bd -literal -offset indent
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24
.Ed
.Pp
Because of the preempt option, when one of the physical interfaces of
host A fails,
.Cm advskew
is adjusted to 240 on all its
.Nm
interfaces.
This will cause host B to preempt on both interfaces instead of
just the failed one.
.Pp
In order to set up an ARP balanced virtual host, it is necessary to configure
one virtual host for each physical host which would respond to ARP requests
and thus handle the traffic.
In the following example, two virtual hosts are configured on two hosts to
provide balancing and failover for the IP address 192.168.1.10.
.Pp
First the
.Nm
interfaces on host A are configured.
The
.Cm advskew
of 100 on the second virtual host means that its advertisements will be sent
out slightly less frequently.
.Bd -literal -offset indent
ifconfig carp0 create
ifconfig carp0 vhid 1 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.1.10/24
.Ed
.Pp
The configuration for host B is identical, except the
.Cm advskew
is on virtual host 1 rather than virtual host 2.
.Bd -literal -offset indent
ifconfig carp0 create
ifconfig carp0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.10/24
ifconfig carp1 create
ifconfig carp1 vhid 2 pass mekmitasdigoat 192.168.1.10/24
.Ed
.Pp
Finally, the ARP balancing feature must be enabled on both hosts:
.Pp
.Dl sysctl net.inet.carp.arpbalance=1
.Pp
When the hosts receive an ARP request for 192.168.1.10, the source IP address
of the request is used to compute which virtual host should answer the request.
The host which is master of the selected virtual host will reply to the
request, the other(s) will ignore it.
.Pp
This way, locally connected systems will receive different ARP replies and
subsequent IP traffic will be balanced among the hosts.
If one of the hosts fails, the other will take over the virtual MAC address,
and begin answering ARP requests on its behalf.
.Sh SEE ALSO
.Xr inet 4 ,
.Xr pfsync 4 ,
.Xr rc.conf 5 ,
.Xr ifconfig 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
device first appeared in
.Ox 3.5 .
The
.Nm
device was imported into
.Fx 5.4 .